In surgery, the ability to determine whether or not a tissue will survive is of paramount importance. This is particularly true in plastic surgery where efforts to develop a reliable method to predict skin flap viability are on-going. It is therefore not surprising that efforts to develop a reliable method to predict tissue viability go back to the very beginnings of plastic surgery. Clinical assessment, based on observations of color, temperature and capillary perfusion have always been, and remain, the basis of good management. However, the clinical signs which arise as a consequence of poor blood perfusion along the flap become evident only after several hours of compromised perfusion. Prolonged and severe deprivation of oxygen and other nutrients to the tissues results in irreversible tissue damage leading to necrosis and loss of tissue. Early, nonsubjective detection of poor tissue oxygenation following surgery increases the likelihood that intervention aimed at saving the tissue will be successful. As a consequence, a variety of methods have evolved over the years to augment clinical judgement. Broadly speaking, these methods fall into two categories: those that are based on an assessment of blood flow within tissues and those that examine the cellular metabolism within tissues.
Early studies depended on the use of pharmacological agents to assess blood flow (Hynes, 1948, Br J Plast Surg 1:159-171; Conway et al, 1951, Surg Gynecol Obstet 93:185-189). It was soon realized that the use of pharmacologic agents gave inconsistent results which were difficult to interpret.
Later, vital dye studies and the use of radioisotopes added to our knowledge (Dingwall and Lord, 1943, Bull Johns Hopkins Hosp 73:129-134; Kety, 1949, Am Heart J 38:321-328). Also, fluorescein dye or other labelled tracer measurements of blood flow were necessarily invasive procedures and the invasiveness of the procedure coupled with prolonged washout times limited the frequency with which these methods could be applied to the site of interest.
More recently, Doppler evaluation of blood flow (Swartz et al, 1988, Plast Reconst Surg 81:149-161), assessment of oxygen transport (Hjortdal et al, 1990, Scand J Plast Reconstr Surg Hand Surg 24:27-30) and sophisticated monitoring of tissue temperature (Kaufman et al, 1987, Ann Plast Surg 19:34-41) held promise. Laser Doppler velocimetry had the advantage of being a non-invasive method. However, it only measured the blood flow within a small volume of tissue, and microvascular heterogeneity rendered the method prone to errors: motion of the probe during measurement and poor probe placement reproducibility both led to poor reproducibility. In addition, the recognition of arteriovenous shunting, where blood flow bypasses the capillary bed resulting in non-nutrient flow, reduced the value of a strictly blood flow-based investigation. Blood flow methods primarily address the issue of oxygen delivery, but provide little information on cellular utilization.
Finally, the most recent advances examine the metabolic status of tissues using magnetic resonance imaging (Cheung et al, 1994, Magn Reson Med 32:572-578) and infrared spectroscopy (Irwin et al, 1995, Br J Plast Surg 48:14-22). Magnetic resonance spectroscopy can provide information on the metabolic status of skin flaps at a cellular level (Cheung, 1994), but the extended measurement times, high costs, and limited portability render the method clinically impractical.
Regarding infrared spectroscopy, U.S. Pat. No. 3,638,640 to Shaw, for example, teaches disposing radiation sources and detectors about the ear of a patient and measuring the intensity of radiation passing therethrough. The logarithms of the detector responses are then combined linearly to yield an indication of oxygen saturation in the blood based on the ratio of concentration of oxyhemoglobin to total hemoglobin in the patient's ear. Clearly, this device is limited in that it provides information only on the oxygenation level of the blood.
U.S. Pat. Nos. 4,223,680 and 4,281,645, both to Jöbsis, teach a method and an apparatus for continuous in vivo monitoring of metabolism in a body organ using near infrared spectroscopy. Specifically, oxygen sufficiency in an organ is measured based on the absorbance characteristics of cytochrome a, a3. However, this apparatus is arranged to measure changes and trends in metabolism and not tissue viability.
U.S. Pat. No. 5,074,306 to Green teaches a method for measuring burn depth comprising administering a fluorescent compound to the skin burn and exciting the fluorescent compound with infrared light. The amount of fluorescence detected at the skin burn compared to the adjacent unburned skin provides an indication as to the depth of the burn. However, as with fluorescein dye and other vital dyes discussed above, this method is invasive and may harm the compromised tissue or the surrounding tissue. Furthermore, the device is used to determine the depth of the burn in the tissue, not the viability of the tissue.
U.S. Pat. No. 5,497,770 to Morcos teaches an apparatus for measuring dynamic oxidative metabolism in compromised myocardium. Specifically, Morcos teaches injecting one or more metabolic substrates in a region of the compromised myocardium and collecting near infrared spectra over time using a probe inserted into the region of interest. The collected data is then used to determine whether a metabolic pathway cascade or a transmembrane ionic potential are intact, which is in turn used to provide an indication as to the viability of the cells. Clearly, this method is too invasive for use with damaged tissue, as it requires both the insertion of a probe below the surface of the tissue and injection of one or more metabolic substrates into the tissue.
Irwin (Irwin, 1995) teaches a method and a device for continuously monitoring concentration changes in oxyhemoglobin, deoxyhemoglobin and total hemoglobin. However, the method is limited in that the device must be mounted onto the surface of the tissue being examined and, as a result, is prone to interference if the probe is removed and subsequently replaced on the suspect tissue. Furthermore, as noted above, the method and the device are designed to measure trends meaning that hemoglobin levels must be continuously monitored in order for useful data to be collected.
It is therefore not surprising that a recent survey amongst microsurgeons showed a lack of uniformity in the use of monitoring devices to assess viability of transplanted tissue (Neligan, 1993, Microsurgery 14:162-164).
The ideal test to predict skin viability should be quick and simple to perform, accurate and reproducible, inexpensive, non-invasive, cause little or no patient discomfort and should not alter the basic physiology of the tissue.